1. Field of the Invention
Preparations of substituted tetrathiafulvalenes from reactions of carbon disulfide with various acetylenic compounds are well know. Of particular interest herein is a one-step method for synthesis of substituted tetrathiafulvalenes from reactions of carbon disulfide with an acetylenic compound under high pressure conditions.
2. State of the Art
Recent findings of the unusual electronic properties of complexes of tetrathiafulvalene (TTF) have generated increased interest in new synthetic routes for preparation of TTF and substituted TTF analogue-compounds. It has been found, for example, that TTF can complex with tetracyano-p-quinodimethane (TCNQ) to form a crystalline charge-transfer salt. This TTF.TCNQ salt, in which TTF is characterized as the electron-donor cation, exhibits metallic properties over a wide temperature range and has reportedly one of the highest electrical conductivities (.sigma..sub.max =1.47.times.10.sup.4 ohm.sup.-1 cm.sup.-1 at 66.degree. K.) of known organic materials [see Ferraris et al, J. Am. Chem. Soc., 95, 948 (1973)].
The superior electrical properties of the TTF.TCNQ salt, a so-called "organic metal", make the salt a particularly likely candidate for many solid-state or physical-electronics applications. In such applications, materials of very high purity are usually required. Known preparations of TTF involve multi-step synthetic routes which typically produce TTF in low yields or in relatively impure form.
One synthetic route to TTF involves deprotonation of 1,3-dithiolium salts accomplished, for example, by coupling of 1,3-dithiolium hydrogen sulfate with a tertiary amine. Suitable thiolium salts for deprotonation to TTF are synthesized by the seven-step method of Klingsberg [Klingsberg, J. AM. Chem. Sec., 86, 5290 (1964)], which method involves a complicated preparation of 1,3-dithiolium hydrogen sulfate starting from thiophosgene and disodium salt of dimercaptomaleonitrile. Another synthetic route, as devised by Wudl et al [Chem. Commun. 1453 (1970)], involves reaction of acetylene, sulfur and carbon disulfide to form 1,3-dithiole-2-thione, which thione compound is oxidized to 1,3-dithiolium hydrogen sulfate; a coupling reaction of this salt with triethylamine in acetonitrile yields TTF.
Increased yields and purity of small amounts of TTF have been reported by the step-wise reduction of commercially available 2-thiomethyl-1,3-dithiolium iodide with sodium borohydride to give 2-thiomethyl-1,3-dithiole as an oil, which oil when treated with fluoroboric acid forms 1,3-dithiolium fluoroborate; this compound on deprotonation with excess amine provides in high purity TTF [Wudl et al, J. Org. Chem., 39, 3608 (1974)].
More suitable to large-scale preparation of TTF is a somewhat simplified version of the Klingsberg Method reported by Melby et al, J. Org, Chem., 39, 2456 (1974). Melby's six-step method requires preparation of a diester, namely, dimethyl 1,3-dithiole-2-thione-4,5-dicarboxylate, which diester is hydrolyzed, under a two-hour reflux condition in the presence of hydrochloric acid and acetic acid, to its corresponding dicarboxylic acid; the diacid is subjected to a pyridine reflux to form a thione which is then converted to 1,3-dithiolium hydrogen sulfate, and thereafter a thiolium coupling salt is made with hexafluorophosphate; TTF is then formed by coupling reaction in the presence of a tertiary amine. Melby also reported synthesis of TTF in small amounts by reacting methyl propiolate, tributylphosphine and carbon disulfide in tetrahydrofuran at -30.degree. C. The resulting diester, namely, 4,4'(5')-bis(carbomethoxy)-.DELTA..sup.2,2' -bi-1,3-dithiole, is subjected to alkaline hydrolysis to form the counterpart diacid, which diacid when decarboxylated provides TTF in an overall yield of about 13 percent based on methyl propiolate starting material.
In U.S. Pat. No. 3,876,662 to Hartzler, substituted TTF compounds are synthesized by reactions of carbon disulfide with various acetylenic compounds bearing electron-withdrawing groups. The starting materials typically are heated at about 100.degree. C. for a period of two to four days, with the reaction carried out in a sealed container under autogeneous pressure. Hartzler discloses obtaining increased yield of substituted TTF compounds with the use of a catalyst provided by an organic acid having pKa less than about five.